The invention concerns a procedure for the determination of the total organic carbon (TOC) content of liquids, in particular that of ultra-pure water, whereby a sample of the liquid under investigation is directed into a reaction chamber and is treated statically batch by batch by the use of UV radiation, for oxidising carbon mainly to carbon dioxide, the sample quantity is then transferred by liquid entering the reaction chamber from the outside to a measuring cell connected to the reaction chamber, where the conductivity is measured and then the carbon content (TOC) is found from the conductivity measurements.
The TOC content of a liquid is the total content of organic carbon. The determination of this carbon content, in particular in ultra-pure water, is of special importance to modern high technologies. At this point, ultra-pure water which is necessary for semiconductor production is intended. However, the pharmaceutical industry also depends upon the reliable monitoring of traces of organic contamination. Even the slightest contamination with hydrocarbon compounds, aliphatics and/or aromatic alcohols etc. is to be avoided at all costs. In the past, methods of measurement and their attendant apparatus have been developed, which were able to determine the content of organic carbon in ultra-pure water over a wide range, from a few ppm (parts per million; 10xe2x88x926) to a few ppb (parts per billion; 10xe2x88x929).
U.S. Pat. No. 3,958,941 describes a procedure whereby with the aid of a dual circuit system the organic components are oxidised in a reaction chamber by irradiation with UV light. The carbon dioxide produced by this procedure is transferred to a measuring cell filled with ultra-pure water, via a separate (air-) duct. The conductivity of the contents of the measuring cell is determined, which increases due to the reaction with the introduction of carbon dioxide.
A method such as this requires the execution of various measurements in advance, in order to calibrate the measuring cell, or the conductivity measuring apparatus used. Apart from that, the accuracy when only a small amount of organic carbon compounds is present is low, as measuring errors caused by the UV radiation cannot be ruled out. Moreover, it is mandatory that the carbon dioxide developed in the reaction chamber be transferred to the measuring cell and dissolved in the ultra-pure water. This type of procedure is not just complicated, but also leads to undesirable sources of error, for example the solubility of the carbon dioxide in the measuring cell changes, depending upon temperature. Apart from this, atmospheric carbon dioxide which unintentionally finds its way into the apparatus, can interfere with the accuracy of measurement.
An attempt at overcoming the above-mentioned drawbacks is made in the European script 0 150 923. The carbon content of a static water sample is determined in such a way that this water sample is placed in a chamber with conductivity sensors and statically held there. A UV lamp, which is mounted on the outside of the chamber, irradiates the water sample until the carbon compounds are completely oxidised. The difference in the conductivities measured before and after UV irradiation should then represent the equivalent of the TOC content. The system described in EP 0 150 923 is completely closed and together with its static measurements, may well prevent the influence of contamination and the introduction of carbon dioxide from the atmosphere. However, the price paid for this is the disadvantage that the UV lamp which is mounted outside of the chamber or measuring cell, has a relatively small angle of illumination, and can deliver very little effective radiation into the measuring cell or oxidation chamber. In addition, a quartz window is fitted as the optically transparent closure of the measuring cell, which corresponds to radiation losses due to absorption and reflection. These losses are in addition to the unavoidable losses caused by the quartz glass envelope of the UV lamp.
Moreover, due to the measuring electrodes which are mounted inside the oxidation chamber or measuring cell, it is necessary that a thick stratum of water awaiting irradiation be present, which exceeds the effective penetration of the UV radiation. Consequently, the organic contents of the static water in the chamber are oxidised only very incompletely and slowly. Although TOC concentrations in the order of ppb can be determined using this apparatus, this leads to undesirably high measuring errors, especially in the lowest measuring range. In fact, oxidation times of up to approx. 10 minutes are necessary, which produce conductivity values which asymptotically approach a limit which has to be mathematically determined. In any case, the conductivity measurement can only be taken while the UV lamp is switched on, whichxe2x80x94as in the case of U.S. Pat. No. 3,958,941xe2x80x94creates considerable interference in the measuring electronics, which have to be present, as UV lamps are usually driven by a high voltage in the order of several hundred volts.
A further disadvantage of EP 0 150 923 is that, due to the relatively large quantity of water contained in the measuring cell or oxidation chamber, the measurement of conductivity reacts slowly, thus displaying a large time constant. Also, due to the design of this apparatus, areas of shadow can form behind the measuring electrodes, which will not be reached by the UV radiation. In addition, the UV lamp will operate at high temperatures the longer it is in use, making not just a heat sink necessary, but also possibly leading to undesirable measuring errors, due to overheating, as the conductivity determined is not just a function of the carbon dioxide concentration in the water, but also of temperature. Consequently it is important to maintain the liquid or water in the measuring cell at as constant a temperature as possible, which is not possible in the case of the present scheme due to the peculiarities of the design, making a rapid temperature measurement, and possibly temperature compensation necessary. In addition, a further (indirect) effect comes into play, which can be explained by the fact that the intensity of illumination of a mercury vapour lamp, regularly used to produce the UV radiation, decreases with rising temperature. In any case, considerable problems are caused by the long oxidation times, which need to be avoided at all costs, in order to increase the accuracy of the measurements.
Above and beyond this, it is known from the German publication 32 23 167 that water can be continuously examined for decomposable organic and/or inorganic carbon compounds. To this purpose, the water under investigation is continuously passed through an irradiation cell, where here the pH value is set to between 7.0 and 7.3. There is a subsequent irradiation of the water under investigation with UV light, and a continuous transfer of the gas mixture which contains the volatile products of decomposition from the irradiation cell to a measuring cell. The above-mentioned volatile products of decomposition are continuously partially extracted from the water under investigation by means of a circulation gas. Then the IR (infrared) absorption of the circulation gas is measured and/or the circulation gas is transferred to a conductivity cell which supports a continuous stream, and partially dissolved there, whereby the conductivity of the water changes. The inorganic carbon compounds which are normally broken down by acids, do not need to be first removed, in order to determine the presence of organic carbon compounds. The procedure for the determination of the organic carbon content is practically the same as described in U.S. Pat. No. 3,958,941, mentioned above, and result in the same disadvantages as described before.
Finally, an apparatus for the production of ultra-pure water is known from the U.S. Pat. No. 5,272,091 which consists of an oxidation chamber set in the main stream, in which the entire quantity of water to be purified is treated with UV radiation as it passes through. Simultaneously, a base value for conductivity is taken upstream of the oxidation chamber in a measuring cell, while downstream of the oxidation chamber in a further measuring cell, a higher value, due to the irradiation is measured. The difference between this and the base value is an indication of the content of organic carbon destroyed by the UV irradiation. Since the values for conductivity can vary with the type of carbon compounds and due to other factors, from time to time the production of ultra-pure water is interrupted and sequentially, batch by batch, several amounts, being the contents of the oxidation chamber, are exposed for various lengths of time to UV radiation. After each irradiation, the main stream of water is temporarily pumped through the oxidation chamber again and a measurement is taken in the downstream measuring cell, and correlated with the length of time of the irradiation. From this set of results, a reference value for the for the rise in conductivity when the entire organic carbon content is destroyed can be calculated, which can then be compared with the rise in conductivity calculated from the measurements taken upstream and downstream of the oxidation chamber during the subsequent treatment of continually flowing water.
This last known procedure suffers from the disadvantage of great inaccuracy, compared to measuring procedures with reaction chambers and measuring cells which are not deployed in the main stream of a system for the treatment of water and need therefore only to take sample quantities, since in the main stream all flow cross-sections have to be large and it would take a very long time if the entire organic carbon were to be destroyed by UV radiation in the large volume of water in the oxidation chamber. The aforementioned problem, that the UV radiation is only effective down to a small depth of water, arises here in an extreme form. This is why, in the case of this known apparatus, the duration of the irradiation is relatively short, in principle the total oxidation of the organic carbon is foregone, instead of which the conductivity reference value is determined by extrapolation, which introduces a further source of inaccuracy. Finally, the accuracy of the measurement procedure in the measuring cell is affected, since its cross-sections are also adapted to the rate of flow of the main stream. Moreover, only a single conductivity value is determined for each batch measured, and not, as the irradiated volume of water is pushed out of the oxidation chamber, a conductivity profile which can be analysed, of the column of water flowing out. The last point will fail, due to the fact that the water, which has undergone irradiation of varying intensity, is mixed within the large flow cross-sections on its way to the measuring cell, so that complete destruction of the organic carbon content, represented by a peak will not occur.
It is the object of the invention to develop the last described procedure so that the carbon content of the liquid under investigation can be determinedxe2x80x94also in the ppb rangexe2x80x94employing a simple construction and high oxidation effect, quickly and reliably with few measuring errors. A specially designed apparatus shall also be created.
In order to achieve this object, the invention suggests that in a procedure of the above mentioned sort, the sample quantity after carbon oxidation with a UV immersion lamp be transferred to a conductivity measuring cell which is connected to the reaction chamber, yet is separate from the reaction chamber, by liquid entering the reaction chamber from the outside, and that here the carbon concentration which is proportional to the carbon content of the liquid be dynamically registered as it flows through the measuring cell. A usual procedure is to clean the reaction chamber and the measuring cell by flushing liquid through themxe2x80x94of course without external energyxe2x80x94before and/or after determination of the carbon concentration. This flushing liquid is usually the liquid under investigation, which is also generally used as the liquid which enters the reaction chamber from the outside after oxidation of the carbon. In any case, defined values for the carbon concentration in the liquid under investigation can be given, before carbon oxidation and/or after the carbon concentration is determined. This should be as small as possible. In the case of this invention, the carbon dioxide concentration in the liquid is determined by means of conductivity measurements. The invention rests upon the concept that dissolved carbon dioxide in a liquid, usually water, reduces the resistance due to the forming of hydrogen ions and carbonate ions, so that the conductivity rises.
The result is that in any case, conductivity measurements can be given, which for the purposes of the invention will be designated as the base value, corresponding to the carbon dioxide concentration in the untreated liquid. This base value is measured as well as the maximum value attained for the carbon dioxide concentration as the liquid enriched with carbon dioxide flows through the measuring cell. Thereafter, the difference between the maximum value and base value, which is proportional to the carbon content of the liquid, is regularly calculated (taking non-linearities in the aforementioned relationship into account).
In summary, the invention uses the fact that the untreated liquid corresponds to a base value of conductivity, which approaches the maximum value for conductivity or carbon dioxide concentration as soon as the volume of liquid which has been treated in the reaction chamber before, leaves the measuring cell due to flow through the system. One can consider the volume of liquid of the sample, which has statically been brought to the oxidation of the carbon with external energy, to be a column of liquid which is xe2x80x9cpushedxe2x80x9d downstream into the connected measuring cell by the liquid entering the reaction chamber from the outside. This treated column of liquid now flows through the measuring cell, where the carbon dioxide concentration, which is proportional to the carbon contentxe2x80x94in this case the conductivityxe2x80x94is dynamically determined. The accompanying measured signal of conductivity increases and subsequently decreases over time, forming a peak, the maximum of which can be determined without difficulty. This is usually done in that the measured signal is tracked over time in a control/analysis unit, connected to the conductivity sensor, that is to say the gradient is determined by differentiation over time. The maximum value of conductivity is arrived at when this gradient reaches a value of zero. This maximum value of conductivity can be set in relation to the base value of conductivity or carbon dioxide concentration by taking the difference. This difference indicates the carbon dioxide concentration, produced by oxidation, of the liquid under investigation. From this carbon dioxide concentration, the carbon content of the liquid under investigation can be determined without difficulty. Yet attention is directed to the exemplary EP 0 150 923 and the article quoted therein, xe2x80x9cA New Approach to the Measurement of Organic Carbonxe2x80x9d by Poirier et al. (American Laboratory, December 1978).
The external energy for the oxidation of the carbon in the liquid under investigation is supplied by a UV lamp which is immersed in the liquid under investigation in the reaction chamber. This UV lamp can be a mercury vapour lamp. Normally this type of source of radiation emits in a wavelength range of between 10 and 380 nm, predominantly between 170 and 260 nm.
First of all, a simple design, according to these inventive measures achieves a high oxidation effect. This is simply explained by the fact that in the preferred configuration, a UV immersion lamp is immersed in the reaction chamber and is in direct contact with the liquid under investigation in the reaction chamber. In this connection, work can be done on small sample quantities leading to small cross-sectional dimensions in the reaction chamber, so that in any case, the cross-sectional dimension can be matched to the depth of penetration of the UV lamp. Apart from this, due to the direct coupling of the UV lamp into the reaction chamber, absorption and reflection losses resulting from a quartz closing window are minimised. This method has the further benefit that auxiliary cooling of the UV lamp is unnecessary, since short oxidation times are attainable, due to the small depth of penetration and high intensity of irradiation. Moreover, the cleaning phases of liquid flushing which precede and follow ensure a stable temperature condition for the UV lamp.
Further, a quick and reliable determination of the carbon dioxide concentration from the conductivity measurements is possible, since this relies only on the determination of the base value and maximum value of the carbon dioxide concentration or conductivity. A measurement of this nature can be done quickly, without any trouble and without serious measuring errors, since background effects have, so to speak, been eliminated. Furthermore, the constancy of temperature of the UV immersion lamp results in a further reduction in measuring errors, since neither a reduction in the intensity of radiation due to temperature, nor a warming of the liquid treated in the reaction chamber is to be expected. It follows that the effects of temperature when measuring conductivity can be ruled out. Finally, it should be taken into account that since the measurements are taken when the UV lamp is switched off, interference in the conductivity sensor, or sensors, as well as in the control/analysis unit is reliably prevented. In addition, the technical properties of the measuring cell can be optimised and no account need be taken of a good transparency to UV radiation. Since the UV lamp is in constant contact with the water or liquid to be gauged, no additional heat dissipation via cooling fins is necessary, so that not only the temperature constancy is improved, but also cost improvements can be asserted. The UV lamp works constantly at its optimal operating temperature, i.e. in the temperature range with the maximum intensity of radiation. Finally, it should not go unmentioned that the UV lamp can be exchanged very easily, since as already stated, it is an immersion UV lamp which is mounted in a reaction chamber which in general is of simple design and independent of the measuring cell. These are the main advantages of the invention.
The subject of the invention is also an apparatus for the determination of the organic carbon (TOC) content of liquids according to claim 5. Advantageous designs of this apparatus are described in claims 6 to 14.